The present invention relates to energy conversion devices and, more particularly, to a combined cycle gas and steam turbine system integrated with a coal gasification plant.
Gas turbines are frequently employed in, for example, electric generating installations, in order to take advantage of their rapid start-up and shutdown capabilities. For example, a gas turbine may be brought up from an inactive condition to full operation, as well as from full operation to a shut-down condition, in a matter of minutes. The simplicity afforded by this rapid start-up and shut-down capability is in contrast to the relatively slow and complex start-up and shut-down of large base-load steam turbines which are more economically maintained in operation for long periods measured, for example, in years. Although flexible in their ability to be started and stopped, gas turbines suffer thermodynamic inefficiency due to the relatively large part of the heat generated by fuel burning therein passing out unused in the exhaust therefrom. An exhaust temperature of, for example, 1030 degrees F. is conventional for a commercial gas turbine. Under normal conditions, a gas turbine generator has a thermodynamic efficiency of about 31 percent. In contrast, the thermodynamic efficiency of a base-load steam turbine power plant is on the order of about 38 percent. This difference in efficiency dictates that a gas turbine used in generating electricity in an electric network be customarily used for relatively short times, principally as a relatively high-cost peaking generation element which is started up only when the base-load apparatus is unable to sustain the system energy usage, and is shutdown as soon as the peak energy usage has passed.
Combined cycle systems include means for recovering the sensible heat available in the gas turbine exhaust for further use. One combined cycle system employs a heat recovery steam generator using gas turbine exhaust heat to generate steam which is then available to a using process such as, for example, a steam turbine. A combined cycle system using a heat recovery steam generator having a high pressure steam turbine followed by an intermediate pressure steam turbine is capable of a thermodynamic efficiency of about 46 percent.
Gas turbines require a clean fuel such as, for example, a liquid gaseous hydrocarbon. Both liquid and gaseous hydrocarbons are expected to become more scarce and expensive. A large quantity of coal is available but, due to the presence of unburned carbon, ash and other contaminants generated during direct use, coal is unsatisfactory for direct use in a gas turbine. Coal gasification may be employed to convert a substantial portion of the hydrocarbon in coal into a clean low-energy or medium-energy gaseous fuel suitable for use in a gas turbine. A preferred coal gasification process employs an oxygen plant to produce pure oxygen. The use of oxygen instead of air in the coal gas plant avoids the presence of nitrogen in the coal gas. Such nitrogen would not only reduce the heating value of the coal gas, but also may contribute to the generation of NOx emissions. After cleaning to remove particulate and chemical pollutants (principally sulfur), the coal gas is burned in the gas turbine.
A coal gasification plant may be integrated with a combined cycle system to produce an integrated plant in which the fuel gas produced by the coal gasification plant is fed directly to the combined cycle system for immediate consumption, or is stored for later consumption. Immediate consumption permits taking advantage of certain sources of thermodynamic efficiency such as, for example, the use of some of the waste heat energy of the coal gasification process to at least partly preheat the fuel gas fed to the gas turbine.
It will be clear to one skilled in the art, the improvement in thermodynamic efficiency obtained in a combined cycle system comes at the expense of increased capital cost. Substantial elements of cost in a combined cycle system are the condenser and a cooling tower which are conventionally required for condensing the spent steam from the steam turbine. Although a non-condensing steam cycle may be used, such a cycle is conventionally relatively inefficient since it requires venting a substantial portion of the spent steam from the steam turbine in order to permit recycling the remaining water in the spent steam. Venting steam wastes a substantial quantity of unused heat with it which may effect a substantial reduction in thermodynamic efficiency.